1. Field of the Invention
The present invention relates to a fixing apparatus for use in image forming apparatus such as an electrophotographic printer and a copying machine.
2. Related Background Art
As a fixing apparatus provided for such image forming apparatus, there has been known an apparatus which fixes an unfixed toner image on a transferring material by passing a nip portion a recording medium, that is, a transferring material such as paper having an unfixed image, that is, an unfixed toner image borne thereon while pressurizing and heating the same, the nip portion being constituted by a fixing body that is a thermal fixing means in the form of a hollow roller or of an endless film to be heated from its inside by a first heating means, and a pressurizing body that is an elastic pressurizing roller in press contact with the thermal fixing means.
Recently, in order to prevent offsets of unfixed toner image to the surfaces of the thermal fixing means and the elastic pressurizing roller, many fixing apparatus have adopted a construction in which potential difference is induced between the surfaces of the thermal fixing means and the elastic pressurizing roller in such a direction that presses the unfixed toner image formed on a transferring material against the transferring material.
As a fixing apparatus adopting a thermal roller fixing method which utilizes a pipe-like metal roller as a thermal fixing means, there is, for example, one having the following construction.
Specifically, the construction is such that electrical potential repelling a toner is induced on the surface of a fixing body, that is, a fixing roller by applying a bias having the same polarity as the toner (unfixed toner image) (when using a negative toner, the bias is xe2x88x92100 V to xe2x88x922000 V) to the core metal of the fixing roller, while a pressurizing body, that is, an elastic pressurizing roller is made to be a moderately resistant roller (surface resistance of 107 xcexa9 to 1012 xcexa9) by dispersing a conductive material in its fluororesin tube, as a surface layer, and elastic layer, and the potential difference between the fixing roller and the elastic pressurizing roller is maintained by connecting a diode to the core metal. The reason to select 107 xcexa9 to 1012 xcexa9 for the surface resistance of the elastic pressurizing roller is that, if the surface resistance is less than 107 xcexa9, there occurs a problem with pressure resistance of the fluororesin tube and, if the surface resistance is more than 1012 xcexa9, the fluororesin tube is negatively charged and there exists no potential difference between the fixing roller and the elastic pressurizing roller, thereby causing offsets.
There is another example of fixing apparatus which is constructed in such a manner that the core metal of the fixing roller is grounded, the surface resistance of the elastic pressurizing roller is made as low as 106 xcexa9 or less, and the potential difference between the fixing roller and the elastic pressurizing roller is maintained by bringing an electrode, such as a brush, to which a bias of low voltage (when using a negative toner, the bias is +100 V to +500 V) is applied into contact with the surface of the pressurizing roller. If the surface resistance of the pressurizing roller is more than 106 xcexa9, potential difference cannot be induced on the surface of the rollers, thereby causing offsets.
In both former and latter cases, the potential difference between the surfaces of the fixing roller and the pressurizing roller is commonly set in such a manner as not to cause offsets under the conditions of low temperature/low humidity.
Thus, inducing potential difference in such a direction that presses the toner against the transferring material has made it unnecessary to apply a parting agent, such as silicone oil, to the surface of the thermal fixing means or to bring a cleaning member into contact with the thermal fixing means and the pressurizing roller, as having done in the conventional apparatus. This causes no accidents such as oil leaking, and moreover, saves the users from having to replace an old cleaning member with a new one.
In recent years, neutral paper has been used as a transferring material for use in image forming apparatus, taking into account the preservability of paper. And as an additive used for bleaching neutral paper, calcium carbonate (CaCO3) has been often used instead of chlorine, allowing for environmental problems chlorine is likely to cause. Since calcium carbonate (CaCO3) provides a high degree of brightness and is low-priced, in some cases, a transferring material may contain up to 25% of calcium carbonate.
However, the use of the transferring material containing calcium carbonate (CaCO3) (hereinafter referred to as calcium-carbonate-containing paper) in the image forming apparatus provided with the conventional fixing apparatus as described above gives rise to a following problem.
When feeding a sheet of calcium-carbonate-containing paper to the conventional fixing apparatus as described above, the transferring material and the metal and plastic members on the sheet-conveying path rub together, which gives rise to paper dust. Calcium carbonate and calcium-carbonate-containing paper dust are positively charged easily due to the friction against metals and plastics; therefore, when applying a bias to the fixing apparatus so as to prevent offsets of negatively charged toner, calcium-carbonate-containing paper dust is adsorbed on the fixing roller, contrary to the intention.
As a result, in the fixing apparatus which are intended to prevent offsets from occurring by applying a bias having the same polarity as toner to the core metal of the fixing roller, there arises a problem as described below.
The surface potential of the fixing roller on the image printing side is canceled out by the calcium-carbonate-containing paper dust, and the negatively charged toner offsets to the fixing roller. Then the problem of xe2x80x9cadhesion of toner to the pressurizing rollerxe2x80x9d arises in which the offset toner together with the calcium-carbonate-containing paper dust transfer and accumulate little by little on the pressurizing roller at the sheet interval (in state where no paper is at the nip portion in the intervals between printings). Although the amount of the toner transferring to the pressurizing roller is very small, when passing several thousands to tens of thousands of sheets of calcium-carbonate-containing transferring material through the fixing apparatus under the conditions of low temperature/low humidity, the toner grows to become flake-like masses which appear on the rear and front surfaces of the image on the transferring material, thereby impairing the image quality.
Further, when using a moderately resistant tube having surface resistance of 107 xcexa9 to 1012 xcexa9 as a surface-layer fluororesin tube of the pressurizing roller, the surface resistance increases with the increasing number of the passing sheets and the surface is negatively charged; therefore, the potential difference between the fixing roller and the pressurizing roller disappears, causing offsets. Particularly when using calcium-carbonate-containing paper, since its surface resistance is relatively high, the surface of the pressurizing roller is likely to be charged negatively.
In the fixing apparatus which are intended to prevent offsets from occurring by grounding the core metal of the fixing roller, rendering the surface resistance of the pressurizing roller as low as 106 xcexa9 or less, and applying a bias of low voltage having the polarity opposite to toner to the surface of the pressurizing roller, there arises a problem as described below.
Due to the increase in carbon content of the surface fluororesin layer of the pressurizing roller, the parting tendency of the roller surface deteriorates, and when passing several thousands to tens of thousands of sheets of calcium-carbonate-containing paper through the fixing apparatus under the conditions of low temperature/low humidity, the problem of xe2x80x9cadhesion of toner to the pressurizing rollerxe2x80x9d arises, thereby impairing the image quality.
As described above, if the surface resistance of the pressuring roller is made low by dispersing conductive materials, such as carbon and metal powders, in the surface fluororesin layer of the pressurizing roller, the surface potential is stabilized and offsets are prevented from occurring; however, since the parting tendency of the roller surface is decreased, when passing sheets of calcium-carbonate-containing paper, the problem of xe2x80x9cadhesion of toner to the pressurizing rollerxe2x80x9d arises. These are on the horns of a dilemma. At the present stage, it is difficult to form a conductive parting layer which satisfies both the low-resistance and the satisfactory parting tendency of the surface of the pressurizing roller.
Accordingly, an object of the present invention is to provide a fixing apparatus which enables a toner to be prevented from adhering to a rotating member having a conductive parting layer on its surface.
Another object of the present invention is to provide a fixing apparatus including a first rotating member, first heating means for heating the first rotating member, a second rotating member for forming a nip together with the first rotating member, voltage applying means for applying voltage to create potential difference between the first rotating member and the second rotating member, and second heating means for heating the second rotating member, in which a recording material with an unfixed image borne thereon is nipped and conveyed at the nip in such a manner as to be allowed to come in contact with the first rotating member, thereby the unfixed image being heated and fixed on the recording material, the second rotating member having a conductive parting layer on its surface.
The other objects of the present invention will become more apparent from the following description.